Ordinarily, picture signals compression-encoded by the MEG (Moving Picture Experts Group) system are recorded on optical disc. In the MEG system, picture signals consisting of about 15 frames called GOP (Group Of Picture) are caused to be set of processing units, and those picture signals are encoded every the GOP.
In one GOP, as shown in FIG. 1, there are I picture (Intra-Picture) (Intraframe encoded picture), P picture (Predictive-Picture) (Interframe forward direction predictive encoded picture), and B picture (Bidirectionally Predictive-picture) (Bidirectionally predictive encoded picture). I picture (picture I2) serves to maintain independence of GOP, wherein the entirety of that picture is encoded. P pictures (pictures P5, P8) are caused to undergo predictive encoding in forward direction from I picture or P picture. In this case, I picture and P picture are encoded in the same order as original picture In addition, B pictures (pictures B0, B1, B3, B4, B6, B7) are caused to bidirectionally undergo predictive encoding from I picture or P picture.
Conversely, in the case of decoding compression-encoded picture signal, picture I2 is decoded by itself as shown in FIG. 1, but pictures except for the picture I2 are not decoded only by their own picture signals.
However, when the first half portion of GOP including I picture is missing, P picture or B picture of the latter half of that GOP fails to be decoded.
For example, picture signals encoded every GOP-1, GOP-2, GOP-3 are assumed to be recorded, as shown in FIG. 2, on recording medium (not shown). Let now consider the case where picture signals from picture B6 of GOP-1 until picture B3 of GOP-3 are rewritten into picture signals of new GOP-N.
As shown in FIG. 3, in the GOP-1X in which pictures B6, B7, B8 are missing, pictures until picture PS including I picture are not missing. Accordingly, pictures except for the missing pictures are decoded in an ordinary manner.
However, in the GOP-3X in which pictures B0, B1, I2, B3 of the first half portion are missing, I picture of that first half portion is also missing. Accordingly, pictures except for missing pictures would also fail to be decoded.
Namely, in the case where when picture signals compression-encoded by the MEG system are recorded on recording medium, compression-encoded other picture signals are further overwritten, there is not any problem if those picture signals are overwritten in GOP units. However, when picture signal of I picture of any other GOP is caused to be missing by the overwritten picture signals (in the case of units except for GOP unit), there took place the problem that as far as picture signals which are not missing fail to be decoded.
In order to avoid such problems, it is conceivable to rewrite picture signals every GOP. However, since those picture signals must be collectively rewritten every about 15 frames, such rewrite operation is very inconvenient. In addition, since picture signals are compression-encoded every respective one GOPs, it is impossible to rewrite those picture signals every pictures.